The present application relates to metal oxide nanoparticles, a production method thereof, and a light-emitting element assembly and an optical material that include the metal oxide nanoparticles.
In nanoparticle-resin composite materials prepared by adding metal oxide nanoparticles such as nanoparticles of zirconium oxide, niobium pentoxide, indium oxide, tin oxide, cerium oxide, hafnium oxide, or tantalum pentoxide to a polymer, their refractive indices can be controlled to be higher than the refractive index nm of the polymer alone. Therefore, such metal oxide nanoparticles serving as an additive for a polymer are useful for designing and producing various optical materials. Examples of optical products in which an increase in refractive index is effective include optical lenses, light control films, Fresnel lenses, antireflective coatings, optical discs, diffusion films, and holographic substrates.
In the metal oxide nanoparticles used in such applications, it is important that the transparency and other optical properties of the nanoparticle-resin composite materials prepared by combining a polymer are not degraded. When the refractive index np of the metal oxide nanoparticles and the refractive index nm of the polymer are different from each other, the transparency of the nanoparticle-resin composite material in which the optical path length is, for example, on the millimeter order markedly depends on the particle diameter of the metal oxide nanoparticles and the dispersibility of the metal oxide nanoparticles in the polymer. When the metal oxide nanoparticles have a large particle diameter, the transparency of the nanoparticle-resin composite material is decreased because the metal oxide nanoparticles scatter light. When the dispersibility of the metal oxide nanoparticles is not satisfactory, the transparency of the nanoparticle-resin composite material is markedly decreased because the metal oxide nanoparticles are agglomerated, and the agglomerated metal oxide nanoparticles strongly scatter or reflect light.
It is known that metal oxide nanoparticles, the surfaces of which are coated with a surfactant, can be synthesized by liquid-phase synthesis. The surfactant on the surfaces of the metal oxide nanoparticles stabilizes the dispersion state of the metal oxide nanoparticles in solvents and prevents agglomeration. Therefore, it is believed that such a surfactant is also effective in stabilizing the dispersion state in polymers (see, for example, Japanese Unexamined Patent Application Publication No. 2005-75723).
Regarding zirconium oxide nanoparticles, for example, a document written by F. C. M. Woudenberg, W. F. C. Sager, N. G. M. Sibelt, and H. Verweij; Adv. Mater. 2001, 13, 514 (referred to as Document 1) describes that amorphous zirconium oxide nanoparticles are usually formed. A method of synthesizing zirconium oxide nanoparticles the surfaces of which are coated with trioctylphosphine oxide and which have a particle diameter of 10 nm or less and excellent crystallinity is reported in a document written by J. Joo, T. Yu, Y. W. Kim, H. M. Park, F. Wu, J. Z. Zhang, and T. J. Hyeon; Am. Chem. Soc. 2003, 125, 6553 (referred to as Document 2). Furthermore, regarding niobium pentoxide nanoparticles, for example, a document written by P. Griesmar, G. Papin, C. Sanchez, and J. Livage; Chem. Mater. 1991, 3, 335 (referred to as Document 3) describes that amorphous niobium pentoxide nanoparticles are usually formed.
According to Document 1, the resulting zirconium oxide nanoparticles are amorphous. Here, from the viewpoint of increasing the refractive index, it is desired that nanoparticles be crystalline. Therefore, annealing to induce crystallization is necessary. However, in order to crystallize the zirconium oxide nanoparticles, annealing at a high temperature of 400° C. or higher is necessary. Consequently, for example, a surfactant is thermally decomposed and the particles are sintered, resulting in problems of degrading the dispersibility and fineness of the zirconium oxide nanoparticles. In the synthetic method disclosed in Document 2, since agglomerated particles that do not disperse are also produced at the same time, the yield of the zirconium oxide nanoparticles having excellent dispersibility is decreased, thus increasing the production cost. Furthermore, in the technique disclosed in Document 3, the niobium pentoxide nanoparticles are more difficult to crystallize than the zirconium oxide nanoparticles. In order to crystallize the niobium pentoxide nanoparticles, annealing at a high temperature of 550° C. or higher is necessary.